Power inverters convert direct current (DC) power to alternating current (AC) power. The power generated by DC generators, such as solar and wind generators, is converted to AC power with an inverter so that it may be fed (or transmitted) into an electric grid. Three level inverter topologies, sometimes called Neutral Point Clamped (NPC) inverters, have three switching states. NPC inverters have small output voltage steps, which may reduce surge voltages and improve the output AC waveform. NPC inverters are often used in higher voltage applications, such as solar panels and wind turbines. NPC inverters operate with a high pulse-width modulation (PWM) index under normal conditions, and may have a power factor that is close to 1.
Power generators may fail by experiencing a brief voltage drop (e.g., a brownout) or a total failure (e.g., a blackout). During a failure, electricity continues to be fed to the grid. In this condition, called low voltage ride through (LVRT), the inverter continues operating through a low voltage situation. During LVRT, the source voltage and the modulation index for the inverter is small. The power factor for the inverter may also decrease, as the power grid begins exhibiting inductive behaviour. During LVRT, inverters may experience large surge currents that strain the electrical components in the inverters.